1. Field of the Invention
The present invention relates generally to a microwave circuit and is directed more particularly to a microwave circuit in which a stop line or micro-strip is used.
2. Description of the Prior Art
In a prior art microwave circuit generally used, as shown in FIG. 1, on an dielectric such as ceramic dielectric substrate 1 made of, for example, alumina or the like, there is formed a microwave conductor i.e. transmission line 2, while on the back surface of the substrate 1 there is formed a conductor 3 to be grounded therethrough. Thus, this prior art microwave circuit is a micro strip line. Further, though not shown, a so-called strip line is used in which the structure of a conductor-dielectric-transmission line is arranged symmetrical with respect to the transmission line.
In a band pass filter of the prior art microwave circuit, for example, micro strip line band pass filter 5 in which at two positions of the main transmission line 2 there are provided stubs 4 of .lambda./2 length (.lambda. is the wave length) in parallel with each other with the distance of .lambda./4 as shown in FIG. 2 (where only the pattern of the transission line is shown), the frequency characteristic of insertion loss becomes as shown by a curve I in the graph of FIG. 3 and the attenuation characteristic thereof is very gentle.
Further, in a band pass filter 6 with the pattern shown in FIG. 4, if the number of elements, which form the band pass filter 6, is increased somewhat, its attenuation characteristic can be made sharp by some extent. In this case, however, there is newly caused such a defect that the insertion loss of the pass band increases.